The present invention relates to the measurement of strong shock waves, such as shock waves generated during an earthquake, volcanic activity, explosives testing, strong impacts or collisions, and nuclear testing. More particularly, the invention relates to an optical gauge or sensor that may be used to reliably measure shock amplitude for yield verification determination, or other purposes, in environments or conditions where conventional shock gauges or pressure sensors are ineffectual.
For purposes of this application, a "shock wave" is a compression wave produced by a sudden change in pressure and particle velocity. If the sudden change in pressure and particle velocity (hereafter "disturbance") is small, the result is an ordinary sound wave. If the disturbance is severe, however, such as occurs with a massive body rushing rapidly through the air or with an explosion, the result is a definite discontinuity called a strong shock wave. The passage of a strong shock wave through a solid or fluid can redistribute atoms, change the levels of electron energy, and alter the internal energy balance.
There is a frequent need in the art to measure strong shock waves. For example, during nuclear underground testing, it is desirable to verify the yield of the explosion, i.e., how much energy was released, in order to verify compliance with applicable international treaties that limit the use and testing of nuclear devices. See, e.g., "Seismic Verification of Nuclear Testing Treaties'" Office of Technology Assessment (Gregory E. Van Der Link, Project Director) (Library of Congress 88-600523, 1988), hereafter "Seismic Verification".
Yield verification measurements are typically made by measuring the time of arrival (TOA) and physical force associated with the shock wave created by the nuclear explosion. Such measurements are made at various points and distances surrounding the point of origin of the explosion. Unfortunately, conventional sensors, such as piezoelectric crystals, are limited in such measurements because the electrical wires used to couple the sensors to appropriate instrumentation apparatus are destroyed or rendered ineffective (e.g., through electrical pickup of noise associated with the blast). What is needed, therefore, is a sensor or gauge that can withstand strong shock waves and reliably signal appropriate instrumentation apparatus of the sensed force.
The measurement of the source yield on underground tests for the purpose of yield verification has been based, in large part, on the use of TOA measurements using a technique known as "Continuous Reflectometry for Radius verses Time Experiments" (CORRTEX), described generally in the aforecited Seismic Verification reference. In recent years, the CORRTEX technique has been augmented by a particle velocity measurement technique, referred to as KRATZ. The KRATZ gauge is a particle velocity gauge that operates on the principle of embedding a static magnetic field generator in the shock medium and measuring its relative displacement during the shock. A magnesium plate with a current coil embedded in it having an overall density of the shock medium is embedded in the shock media. A magnetic field is established around the current coil prior to shock arrival. Nearby are placed pickup coils that are stationary relative to the magnesium plate displaced during shock arrival. These coils detect a change in magnetic field intensity as the plate moves and thereby can determine the motion of the plate. Since the plate is well matched to the shock medium, the plate can be assumed to be at the particle velocity at that point. These measurement techniques generally assume that valid data, e.g. noise-free data, has first been obtained. Unfortunately, valid data cannot always be obtained due to the problems mentioned herein.
For example, when making international yield verification measurements, there is a requirement that an anti-intrusion switch be coupled to the seismic sensor or gauge to transiently disconnect the gauge prior to zero time. (Note: "zero time" is the precise moment of the detonation that creates the disturbance that causes the shock wave.) Unfortunately, most sensors known in the art for sensing a strong shock wave, e.g., flat pack stress gauges, are "active" gauges, meaning that they require operating power before, during and after sensing the shock wave. In addition to the electrical noise pickup problems that accrue whenever operating power is coupled to the active gauge over conventional wires, the use of active gauges is not particularly amendable to the use of anti-intrusion switches. Hence, what is needed is a purely passive shock gauge that does not require any operating power before, during, or after a shock measurement, and which is thus more amenable to the use of anti-intrusion switches.
Additionally, prior art shock gauges, e.g., flat pack stress gauges, only provide an output electrical signal indicative of the magnitude of the sensed force, and the time at which the force was sensed. Unfortunately, the transmission of this signal to a remote location where the signal can be processed results in a loss of signal strength due to the normal losses associated with an electrical transmission line or cable. associated with the impact of the shock wave, in addition to pressure and TOA data. Such other data may include, for example, thermodynamic data associated with the medium as the shock wave passes therethrough. Heretofore, other sensors adapted to sense the other parameters of interest, e.g., temperature, have had to be employed if such data was to be collected. The use of such other sensors in an environment subjected to strong shock waves is subject to the same problems as is the use of active shock gauges, as described above. Therefore, it would be desirable if a single passive gauge could be employed that provided all or most of the data of interest, e.g., pressure, TOA, and thermodynamic/hydrodynamic data, and if such data could be transmitted to a location removed from the gauge location without appreciable lose in signal strength, and without being susceptable to electrical noise.
One further problem associated with the use of any gauge is the problem of "gauge inclusion". Gauge inclusion relates to the problem of having the gauge itself influence the measurement. For example, if a normal-sized, mercury bulb, thermometer is inserted into a thimble full of water, the temperature of the thermometer prior to the insertion will likely raise or lower the temperature of the water, depending upon the initial temperature of the thermometer, thereby providing a "false" temperature reading of what the water temperature was prior to the insertion. Similarly, if a shock gauge is placed in a medium through which a shock wave is to be measured, the shape, size and material of the shock gauge can influence the manner in which the shock wave propagates, just like a boulder in a creek diverts the flow of water in the creek. A flat pack stress gauge, for example, typically includes a flat plate designed to be positioned perpendicular to the direction of propagation of the shock wave, thereby creating a potentially significant gauge-inclusion interference. Further, such a flat pack stress gauge is made from a material different than that of the material through which the shock wave to be measured is propagating. Hence, the problem of gauge inclusion with a flat pack stress gauge is heightened. What is needed, therefore, is a shock wave gauge that is made from the same or similar material as the medium through which the shock wave is propagating, and that can assume a shape so as to minimize interfering with or altering the shock wave propagation, thereby minimizing the problems associated with gauge inclusion.